Learning Forum

[Gorilla Koala]

Hello,

I'm trying to simulate acoustics using ANSYS Fluent. (Acoustic) Courant number is one of the criteria to decide the timestep size for an explicit calculation. But is it still imporstant for an implicit calculation?

[SRP]

Hi,

The Courant–Friedrichs–Lewy or CFL condition is a condition for the stability of unstable numerical methods that model convection or wave phenomena.

Implicit is unconditionally stable, and also lacks a CFL condition.

For more details, please refer to fluent theory guide: 24.5.4. Steady-State Flow Solution Methods (ansys.com)

If you are not able to access the link, please refer to this forum discussion: https://forum.ansys.com/forums/topic/using-help-with-links/#latest

Thank you

[DrAmine]

For direct CAA CFL condition is important as there you will anyhow use LES or SRS methods/models. You will aim there to use a time step which can capture the highest frequency in your case. If not direct CAA or you want to use Acoustic Analogy then URANS can be used (with compromise on acuracy) and there the run might be less computational expensive.

[Gorilla Koala]

 

Hi DrAmine,

Thank you for the clarification. I have additional questions. 

 

• Is there any criteria of CFL number? I’m currently setting the time step size that LES converge in 10 iterations each time steps, because the ANSYS manual recommends. At this time, the CFL number of the fluid flow is small enough (<<1), but the acoustic courant number is around 3-4 because the speed of sound is exteamly high. The CFL number must be less than 1?
• How does Ffowcs Williams Hawkings model calculatite the acoustic pressure at each receiver? I could not understand why ANSYS can set receivers outside the domain. 

 

Thank you!

 

 

[Gorilla Koala]

Hi SRP,

Thank you for your reply!!

[DrAmine]

Decouploing the soudn waves from the solution of the conservation equations allows the Ffowcs Williams Hawkings Model to calculate acoustic pressure even at receivers located outside of the domain.